Deerfield registers late case of West Nile virus in a mosquito


Staff Writer

Published: 10-13-2023 12:42 PM

DEERFIELD — While it’s late in the season, a mosquito in Deerfield has tested positive for West Nile virus for the first time since 2019.

A mosquito behind the visitors center in Old Deerfield tested positive for the virus on Oct. 5. The town is urging residents to take additional precautions as they wait for cold weather to drive the insects away for the season.

Precautions include using insect repellant, wearing long clothing that covers exposed skin, avoiding outdoor activities around dusk and dawn, repairing broken door and window screens, and emptying yard containers of standing water, according to the Massachusetts Department of Public Health.

Selectboard and Board of Health Chair Carolyn Shores Ness said officials are addressing areas around town using bacillus thuringiensis israelensis, also known as Bti, a bacteria that produces toxins that only affect mosquito larvae. The bacteria poses no risks to humans, according to the U.S. Environmental Protection Agency.

“This is the importance of surveillance and trapping,” Shores Ness said. “This would have been the third year with no disease. It’s very disappointing that we couldn’t squeeze by this year, but it’s late in the season, so I feel like it’s not a threat.”

West Nile virus is the leading cause of mosquito-borne illness in the U.S. Although there is no vaccine or treatments available to fight the virus, the vast majority of people infected show no symptoms. One in five will develop minor symptoms, such as a fever, and one in 150 people will develop a serious case, according to the Centers for Disease Control and Prevention.

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Due to the cooler weather over the last few weeks, Shores Ness said mosquito populations are already down and, with cold weather just a few weeks away, there should be few worries, if any.

Deerfield’s case of West Nile virus is the first in Franklin County, although Belchertown, South Hadley, Northampton and Amherst all registered cases in August and September. The Department of Public Health has designated all of Franklin County as being at “low” risk for the disease, but has put several Hampshire County communities, including Hadley, Northampton and Granby, as “moderate” risk.

For more information about West Nile virus, visit the Department of Public Health’s dashboard at

Chris Larabee can be reached at or 413-930-4081.